This invention relates to apparatus for detecting leaks of molten plastics material in an injection moulding machine, in particular, leaks due to misalignment or wear of the nozzle and the mould in injection moulding of plastics, but also in hot runner injection systems or leaks of molten plastics material at any location or locations where rapid detection of leaking material is necessary to avoid machine down time.
A problem arises in injection moulding with leakage at the interface of the mould and the nozzle of the injection moulding machine. Leakage arises if the nozzle and the sprue bush where the nozzle makes contact with the mould are misaligned or if wear has occurred to the nozzle or the sprue bush opening up an unacceptable clearance gap.
The leakage of molten plastics material under pressure has many harmful effects and may occur as a slow creep or "drool" or as a rapid ballooning of sprue from the nozzle region. A decrease in the level of pressure inside the mould reduces the quality of the product being moulded by causing shrinkage. The leaking drool can short out heater bands, damage wires and terminals and destroy insulation blankets around the injection nozzle. The cost of spare parts and down time during repair is very high. There is also the increased risk to personal safety of machine operators. The solidified drool has an irregular surface and may have to be burnt off with a blow-lamp giving rise to smoke and poisonous fumes. It will be appreciated that the hot plastics material solidifies very quickly.
There have been previous attempts to deal with the problem. One prior device supplied by Insul-Vest Inc. comprises a protective jacket which covers the nozzle and heater bands of the injection moulding machine such that when leakage occurs the drool spreads over the jacket and is prevented from spreading onto the rest of the machine. The jacket is made from material which allows the leaked drool to be removed easily. This device only seeks to limit the damage caused by the leakage. It ignores problems of material wastage and loss of pressure inside the mould causing subsequent deterioration in quality.
Another approach, disclosed in U.S. Pat. No. 4,921,416, is to use a thermocouple which is positioned adjacent to the sprue bush and nozzle interface to detect abnormal changes in temperature caused by leakage. An electronic control unit linked to the thermocouple is adapted to give an alarm when a level of temperature outside the normal operating range is sensed. This is not a satisfactory solution either because the temperature around the nozzle during fault-free operations (when there is no leakage) is not necessarily constant. The temperature can vary depending on the type of plastics used, the length of time during which the machine is in continuous operation and on ambient conditions.
Another approach is to use a proximity sensor to detect the presence of leaked material at a specified location, and this is the approach which is favoured in the present invention. Very many electronic, optoelectronic and electromagnetic switching devices are known to detect the presence or proximity of moving objects on a production line. The present invention seeks to use a simpler, fail-safe method of detection which relies on sensing small changes in air pressure in a pneumatic system which responds quickly to the presence of leaked material adjacent to an air outlet. Pneumatic bleed sensors are known as proximity sensors, but the present invention provides a system of leakage detection providing a rapid response and an automatic alarm and shut down function as well as a fail-safe operation, not hitherto available.